1. Field of the Invention
The present invention relates to the field of paper web drying and in particular to the directing and heating of air in a Yankee Impingement Hood of the type known in the art.
2. Description of the Related Art
A Yankee hood is an air distribution and drying system typically used in the paper industry. A Yankee hood is typically installed over and spaced-apart from a portion of the circumferential surface of a rotatable cylinder. The drying air is heated and pressurized in the system and is then supplied to the Yankee hood dryer where it passes through nozzles at high velocity and impinges on the moving, drying web. The spent air is then collected in the dryer and returned to a recirculation system. Some of this spent air is exhausted, but the majority of it is recirculated to conserve heat.
The heat which is transferred from the impingement air from the nozzles to the paper increases the temperature of the paper to its equilibrium drying temperature; evaporates the water from the paper; and increases the temperature of the paper above the equilibrium temperature after the surface water has been evaporated.
Increasing production rates called for in the industry today result in demands for higher and higher evaporation rates. Achieving evaporation rates considerably higher than those currently available must be realized largely through improvements to the Yankee hood system. In a Yankee hood, the evaporation is driven largely by convection heat transfer, brought upon by the effect of impinging jets of hot air and radiation heat transfer. Effectiveness of hood evaporation largely depends on geometry of impingement air, properties of impingement air, and temperature.
A problem found in current drying systems involves the uneven cross-machine movement and temperature profiles of the impinging air. Temperature profile problems can, and often do, originate at the crescent header and nozzle box. This is more pronounced at higher operating temperature. In particular, it is common for the temperature profile of the air to vary dramatically—often very high temperatures are found directly below the crescent headers and lower temperatures at the troughs between crescent headers. An uneven temperature profile will lead inevitably to a lack of uniformity in the drying process, which can cause sheet breaks and increase production costs and maintenance of the paper-drying machines.
Accordingly, there is a need in the art for an improved Yankee-type drying system that provides a uniform temperature profile through the nozzle box. There is a further need in the art for an apparatus that can be adapted to fit into existing Yankee-type drying systems in order to provide temperature uniformity. Finally, there is a need in the art for a more reliable and cost-effective process for drying paper webs.